The manufacture of wood materials in refiners under optimal conditions permits better qualities than does stone grinding production. But thermal treatment or thermal and chemical treatment of the wood is required prior to defibration. The purpose of such preliminary treatment is to soften the lignin, thereby reducing the energy needed for the release of the fibers from the tissue and producing breaking points in the area of the primary wall and S1. The resultant fiber surfaces are rich in carbohydrate and therefore are well qualified for the formation of hydrogen bridges between the surfaces of these fibers. The temperatures to be applied in the preliminary thermal treatment are between 125.degree. and 150.degree. C. In the case of a treatment time of a few minutes, the above-mentioned aim of lignin plastification is to be reached, but it is not to be so extensive as to result in separation of the fibers in the middle lamella area, which would result in an intact fiber but it would have a hydrophobic lignin coating on the surface. Higher temperatures or longer treatment also have the disadvantage that the lignin structure is changed by condensation reactions and the fibers darken considerably.
By sulfonating the wood at the breaking points a controlled defibration of the wood is achieved, loss of whiteness is prevented and a more hydrophilic lignin is produced at the later fiber surface. The production of more flexible fibers is to be considered as an additional positive aspect of sulfonation.
The energy needs for the isolation of fibers from the wood tissue are diminished by a thermal or chemical pretreatment of the wood. For the production of high-quality fiber materials for paper and linerboard production, however, they have to be additionally defibrillated. In this case wall layers or fibrils are stripped from the surface of the fibers by mechanical action, thereby increasing the specific surface area of the fibers and thus improving their bonding capacity and their flexibility. Such processes are described extensively in "Pulp and Paper Manufacture," vol. 2, Mechanical Pulping, Tappi, Atlanta 1987.
In comparison to the stone grinding process the power requirements in all refiner wood pulp processes are significantly higher. In the stone grinding process the defibering energy is delivered directly to the wood layer in direct contact with the stone surface. In refiner processes the energy transfer is less controlled, since energy is consumed in the acceleration of the pulp, in the rubbing of the wood particles on one another and on the disks, in the forming of the particles and in the fluid friction. In the stone grinding process the forces are always applied transversely of the fiber direction, where the wood has less strength. Since the fibers of the chips of wood in the refiner are not always aligned parallel to the centrifugal force, the energy expenditure on defibration is in this case higher. The thermal and chemical pretreatment can lower the energy needed for releasing the fibers from the wood tissue, but the total energy required for the production of a more or less thoroughly defibrillated wood pulp does not diminish, since the fibers have been made more flexible by the treatment, and can escape the action of the grinding segments of the refiner, so that a more controlled defibrillation becomes possible, but it requires more stressing and relieving processes.
If approximately 1500 kWh/t has to be expended for a high-quality softwood stoneground pulp, thermomechanical pulp (TMP) requires about 2000 and chemithermo-mechanical pulp (CTMP) 2500 kWh/t.
For the production of high-quality wood pulps, a sulfonation of the lignin is necessary, as already mentioned. This is usually performed by using sodium sulfite in an alkaline medium, since a swelling of the fiber also takes place simultaneously, which creates good conditions for the defibration that follows. A sulfonation reaction also takes place in the acid pH range, and the lower the pH is, the faster it goes. However, competing condensation reactions of the lignin are also promoted by low pH values. Lignosulfonates with a high degree of sulfonation are insoluble in water and therefore reduce the fiber yield. On the other hand, acids attack the carbohydrates, depolymerize them and lead to weakening of the fiber bond.
The high energy requirements, especially of the CTMP pulps, limits their production to countries with low energy prices. Future developments in the field of wood pulp manufacture is therefore dependent substantially on the energy requirements of the process. A definite reduction of the energy input appears to be essential.